osd/scrub: do not attempt to read past the end of an object

osd/scrub: do not attempt to read past the end of an object

When performing deep scrubs, the scrubber reads object data
in strides. Existing code uses a short read to detect the end
of the object (and if the object size is a multiple of the
stride - an extra read is performed, which returns 0 bytes).

The proposed change is to avoid such extra read attempts,
by using our knowledge of the object size.

Also - some minor code cleanups in the relevant function.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>